Oscillator system, method of providing a resonating signal and a communications system employing the same

ABSTRACT

An n th -order oscillator system for providing a resonating signal, a method of generating a resonating signal and a communications system. In one embodiment, the n th -order oscillator system, n being greater than two, includes (1) an amplifier configured to provide an intermediate signal and (2) a feedback loop including an n th -order complex LC tank and configured to generate the resonating signal by feeding back a complex-filtered form of the intermediate signal to the amplifier.

This application is a Continuation of application Ser. No. 10/924,220filed Aug. 23, 2004 now U.S. Pat. No. 7,330,082.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to oscillator systemsand, more specifically, to an oscillator system having complexfiltering, a method of providing a resonating signal and acommunications system employing the same.

BACKGROUND OF THE INVENTION

An oscillator is an electronic device that uses an amplifier withpositive feedback to generate a signal. The output of the amplifier isfed back, in phase, to an input of the amplifier to regenerate andsustain the signal. Oscillators are employed in a wide array of devices,such as computer's and wireless transceivers.

In a wireless transceiver, such as a Radio Frequency (RF) communicationssystems, one or more oscillators provide the signal to a transmitter forupconverting (modulating) to an RF signal and to a receiver fordownconverting (demodulating) from an RF signal. Modulation accuracy ofthe transceiver is essential to minimize bit error rate (BER) duringcommunications. This results in challenging noise requirements for theoscillator(s), including stringent specifications for close-in andfar-out phase noise.

The majority of oscillators used in RF communications systems areimplemented using a single inductor-capacitor (LC) resonant circuit(“tank”) with a single negative-resistance stage. Both single-ended,differential and quadrature oscillators are designed based on thisprinciple for which the single LC tank provides only a second-orderfiltering to the noise power injected into the oscillator. In theseoscillators with low order filtering, a low quality factor (Q) of the LCtank results in large power consumption and poor phase noiseperformance. Thus, a high LC tank Q, available headroom of a powersupply and current consumption are needed for an oscillator to satisfyexisting RF communications standards.

As a whole, however, the wireless communications industry is moving inthe opposite direction regarding component Q as newer complementarymetal-oxide semiconductor (CMOS) process technologies (also calledprocess nodes) create products having an inferior LC tank Q and powersupply operating margin. In addition, competition in the industrydemands wireless products with lower current consumption so that amobile terminal can last longer with present battery technology.

Thus, present oscillators typically require a high-Q inductor and ahigh-Q capacitor to meet stringent phase noise requirements demanded bythe industry and communications standards. Future CMOS process nodes,however, will continue to decrease the Q of the LC tank resulting inphase noise worsening. Accordingly, additional fabrication costs will berequired to produce high-Q tanks. In addition, increased quality demandsfor wireless applications place even more stringent phase noiserequirements on the industry.

Accordingly, what is needed in the art is an oscillator having a lowphase noise that satisfies wireless communications standards. Morespecifically, what is needed is an oscillator that can be implementedusing present and future CMOS process technology that satisfies lowphase noise requirements for stringent RF communications standards.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides an n^(th)-order oscillator system forproviding a resonating signal, a method of generating a resonatingsignal and a communications system. In one embodiment, the n^(th)-orderoscillator system, n being greater than two, includes (1) an amplifierconfigured to provide an intermediate signal and (2) a feedback loopincluding an n^(th)-order complex LC tank and configured to generate theresonating signal by feeding back a complex-filtered form of theintermediate signal to the amplifier. An n^(th)-order oscillator systemhas an associated LC tank of an n^(th)-order impedance that providesn^(th)-order filtering of the resonating signal.

The present invention provides an improved LC-tank-based oscillatorsystem employing complex filtering that satisfies stringent phase noiserequirements. The novel oscillator system obtains an improved oscillatorphase noise performance even when implemented in a process with alimited Q-factor of an inductor and/or a capacitor while requiringlimited or no analog extensions. Thus, the disclosed oscillator systemcan be embodied in a deep sub-micron CMOS process node and still satisfyphase noise requirements of present and future applications.

The present invention advantageously recognizes a cooperativearrangement of two LC tanks provides superior complex filtering of noisepower associated with an oscillator compared to only the second-orderfiltering provided by the single LC tanks of conventional oscillators.Thus, the complex filtering of the present invention provided by thecomplex LC tank, results in a reduction of phase noise compared toexisting single LC tank oscillators.

Advantageously, the present invention discloses a novel oscillatorsystem that can have lower LC tank Q compared to conventionaloscillators and still satisfy stringent communication standards. In anembodiment to be discussed, the LC tank oscillator system includes twoLC tanks that are actively coupled to form a complex LC tank thatprovides 4^(th)-order complex filtering and relaxes LC tank Qrequirement. Complex filtering is defined for the purposes of thisinvention as a filtering circuit having complex numbers in thedenominator.

In another aspect, the present invention discloses a method of providinga resonating signal including (1) providing an intermediate signalemploying an amplifier and (2) generating said resonating signal byfeeding back a complex-filtered form of the intermediate signal to theamplifier through a loop that includes an n^(th)-order complex LC tank.

In yet another aspect, the present invention provides a communicationssystem including (1) a transmitter, (2) a receiver and (3) ann^(th)-order oscillator subsystem for providing a resonating signal tothe transmitter and the receiver, n being greater than two. Then^(th)-order oscillator subsystem includes (3A) an amplifier configuredto provide an intermediate signal and (3B) a feedback loop including ann^(th)-order complex LC tank and configured to generate the resonatingsignal by feeding back a complex-filtered form of the intermediatesignal to the amplifier.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment of a communicationssystem constructed according to the principles of the present invention;

FIG. 2 illustrates a block, diagram of an embodiment of an oscillatorsystem constructed according to the principles of the present invention;

FIGS. 3A-3D Illustrate graphs representing phase plots of a resonatingsignal generated by the oscillator system of FIG. 2 indicating a singleresonating frequency; and

FIG. 4 illustrates a flow diagram of an embodiment of a method ofgenerating a resonating signal carried out according to the principlesof the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a block diagram of anembodiment of a communications system, generally designated 100,constructed according to the principles of the present invention. Thecommunications system 100 includes an antenna 110, a duplexer 120, areceiver 130, an oscillator subsystem 140, an intermediate frequency(IP) modem 150, a digital signal processor (DSP) 160 and a transmitter170. In addition to the illustrated components, one skilled in the artwill understand that the communications system 100 may also includeother components that are typically employed in conventional wirelesscommunications systems, such as, a power amplifier, a power manager andan application processor. Additionally, the present invention, appliesto communications systems that may not include each component that isillustrated. For example, a communications system may be a downlinksystem that does not include a transmitter, such as, in a GlobalPositioning System (GPS) receiver.

The communications system 100 is a wireless communications system thatcan be embodied as an RF transceiver. The antenna 110, duplexer 120, IFmodem 150 and the DSP 160 may be conventional components typicallyemployed in a conventional wireless communications system. The receiver130 and the transmitter 170 may also be a conventional receiver andtransmitter. The receiver 130 includes a low noise amplifier (LNA), anRF filter, an RF mixer, an IF filter and an IF amplifier. Thetransmitter 170 includes an IF filter, an RF mixer, an RF filter and apreamplifier.

In a receive mode, the antenna 110 receives an RF signal that isfiltered by the duplexer 120 to attenuate transmit band signals. Thereceiver 130 amplifies the filtered signal employing the LNA and the RFfilter. The RF mixer downconverts the RF signal to an IF using aresonating signal generated by the oscillator subsystem 140. The IFmodem 150 then demodulates the IF signal and outputs in-phase andquadrature-phase data for the DSP 160. The DSP 160 reconstructs theoriginal message that was transmitted from the in-phase andquadrature-phase data. The reconstructed original message is then sentto a speaker or, for example, a digital media such as an MP3 player.

When the communications system 100 is in a transmit mode, an input voicemessage from a microphone is received and processed by the DSP 160 togenerate in-phase and quadrature-phase data streams. The IF modem 150then modulates the data streams with in-phase and quadrature phasesignals to produce an IF signal. The transmitter 170 filters the IFsignal and mixes the IF signal with a resonating signal from theoscillator subsystem 140 to generate an RF signal. The RF signal is thenfiltered, amplified by the power amplifier and fed to the antenna 110through the duplexer 120 that attenuates the spurious signal level.

The oscillator subsystem 140 is an n^(th)-order oscillator with n beinggreater than two. The oscillator subsystem 140 is embodied using CMOStechnology. In one embodiment, the oscillator subsystem 140 isimplemented in a deep-submicron CMOS node. In some embodiments, theoscillator subsystem 140 may be a 4^(th)-order oscillator system asillustrated in FIG. 2.

The oscillator subsystem 140 includes an amplifier 141 and a feedback,loop 142. Since amplifiers contribute a significant amount of noise tooscillators, the present invention advantageously includes onlyamplifier 141 in the oscillator subsystem 140. The amplifier 141 isconfigured to provide an intermediate signal and may be a conventionalamplifier commonly employed in LC tank based oscillators. The feedbackloop 142 includes an n^(th)-order complex LC tank 143 and is configuredto generate the resonating signal by feeding back a complex-filteredform of the intermediate signal to the amplifier 141. The output of theoscillator subsystem 140 could be the intermediate signal or almost anysignal in the feedback, loop 142, preferably after buffering with a highinput impedance amplifier.

Previously, a concern of using a complex LC tank in an oscillator wasthe potential of having multiple resonance (i.e., multiple oscillationfrequencies). The complex LC tank 143, however, can advantageously beused to provide high-order filtering without generating multipleoscillation frequencies. Accordingly, the oscillator subsystem 140 isadvantageously configured to generate the resonating signal at a singleoscillating frequency.

The complex LC tank 143 includes a first LC tank 144 and a second LCtank 145 that are configured to generate the resonating signal. Thefirst and second LC tanks 144, 145, act as a voltage divider to providethe complex-filtered signal back to the amplifier 141. The first andsecond LC tanks 144, 145, may include conventional components.Typically, the first and second LC tanks 144, 145, are actively coupledtogether. Actively coupled is defined as electrically connected, suchas, for example, hardwiring the first and second LC tanks 144, 145,together. In other embodiments, the complex LC tank 143 may include morethan two LC tanks.

Turning now to FIG. 2, illustrated is a block diagram of an embodimentof a 4^(th)-order oscillator system, generally designated 200,constructed according to the principles of the present invention. Theoscillator system 200 includes an amplifier 210 and a feedback loop 220.The feedback loop 220 includes a complex LC tank 230 that has a first LCtank 240 and a second LC tank 250. The oscillator system 200 isadvantageously embodied using CMOS technology. Of course, one skilled inthe art will understand that the oscillator system 200 can be embodiedusing other technologies.

Each of the components of the first and second LC tanks 240, 250, arepassive components, such as inductors, resistors and capacitors, thatare well known in the art. The first LC tank 240 includes a resistancer1, an inductance L1 and a capacitance C1. The second LC tank 250includes a resistance R2, an inductance L2 and a capacitance C2. In theoscillator system 200, r1 and R2 represent a parasitic resistance. Inthe present invention, the lower-case “r” represents series parasiticresistance and the upper-case “R” denotes a parallel equivalentparasitic resistance.

The 4^(th)-order oscillator system 200 provides a resonating signal fordevices, such as, the communications system 100 of FIG. 1. The amplifier210 may be a conventional amplifier configured to provide anintermediate signal. The feedback loop 220 is configured to generate theresonating signal by feeding back a complex-filtered form of theintermediate signal to the amplifier 210. The amplifier 210 employs thecomplex-filtered feedback signal and an input signal to provide theintermediate signal.

The impedance of the complex LC tank 230 can be represented by Equation1 below:

$\begin{matrix}{{Z(\omega)} = \frac{s^{2}L_{2}C_{1}}{{s^{4}L_{1}L_{2}C_{1}C_{2}} + {s^{2}( {{L_{1}C_{1}} + {L_{2}C_{2}} + {L_{2}C_{1}}} )} + 1}} & ( {{Equation}\mspace{20mu} 1} )\end{matrix}$assuming, to simplify discussion, parasitic resistance r1=r2=0, i.e.,R2=r2(1+Q_(L2) ²)=infinity, where Q_(L2) is Q of the second LC tank 250.Thus, instead of an a 2^(nd)-order impedance as in many LC tanks ofconventional LC-tank oscillators, the complex LC tank 230 has animpedance with a 2^(nd)-order equation in the numerator and a4^(th)-order equation in the denominator so that the magnitude of theimpedance rolls off from its peak at a greater rate along both positiveand negative offset frequencies.

The complex LC tank 230 provides a 4^(th)-order impedance for the4^(th)-order oscillator system 200 that provides complex filtering toreduce noise and thus provide an improved resonating signal (v₀). Morespecifically, the first and second LC tanks 240, 250, are configured toattenuate noise from the amplifier 210 by impedance division. Thus, thefirst and second LC tanks 240, 250, reduce the main noise contributor,amplifier noise, to lower total noise at the feedback point v_(o).Further phase noise improvements can be realized by combining theoscillator system 200 with LC tanks having a high impedance.

The oscillator system 200 provides an improved resonating signal thatcan be implemented using deep-submicron CMOS technology. A comparisonbetween oscillators using other LC tank topologies and the improvedphase noise performance of the oscillator system 200 is illustrated byTABLE 1. TABLE 1 represents a summary of simulations for phase noise at20 MHz offset from 3.25 GHz carrier. The simulations were, conductedusing identical circuit parameters and different LC tanks.

TABLE 1 Simulations for Phase Noise at 20 MHz Offset from 3.25 GHzCarrier TYPES OF LC TANK PHASE NOISE (dBc/Hz) Conventional −150.16Transformer Based Complex LC −151.47 Tank LC Tank Without Complex−146.95 Feedback Complex LC Tank of FIG. 2 −160.35 Complex LC Tank ofFIG. 2 −164.85 having a high impedance

In addition to the improved phase noise performance, the resonatingsignal provided by the oscillator system 200 is at a single oscillatingfrequency. This is illustrated by the graphs of FIGS. 3A and 3B usingthe following parameters: L1=L2=1 nH, C2=1.9 pF, Q_(L)=12, Q_(C)=80. Z₁and Z₂ are the respective impedances of the first LC tank 240 and thesecond LC tank 250. C1 is varied from 1 to 3 pF resulting in phase plotsfor Z_(open)=Z₂/(Z₁+Z₂) and Z_(close)= Gm(Z₁+Z₂)/(1−Gm(Z₁+Z₂)Z_(open))=Gm(Z₁+Z₂)/(1−GmZ₂) are shown in FIGS. 3Aand 3B, respectively.

FIG. 3A illustrates that only one point exists with phase=0° forZ_(open), i.e., no second resonance. FIG. 3B shows that the phase ofZ_(close) crosses 0° and 360° only once indicating that oscillation willbe sustained as long as the amplifier provides enough energy to cancelthe energy loss due to r1 and R2. Similarly, as illustrated in FIGS. 3Cand 3D, no second resonance exists when sweeping C2 between 1 and 3 pFand keeping C1−1.9 pF.

Turning now to FIG. 4, illustrated is a flow diagram of an embodiment ofa method of generating a resonating signal, generally designated 400,carried out according to the principles of the present invention. Themethod 400 is triggered by an intent to generate a resonating signal ina step 405.

After starting, an input signal is provided to an amplifier in a step410. The input signal may be an input voltage. The amplifier may be aconventional amplifier typically employed in oscillators associated withRF transceivers. In one embodiment, only one amplifier is employed.

After providing the input signal, an intermediate signal is providedemploying the amplifier in a step 420. The intermediate signal is anoutput of the amplifier. After it is provided, the intermediate signaltraverses through an n^(th)-order complex LC tank in a step 430 with ngreater than 2. In one embodiment, n is four. The complex LC tank maycomprise two LC tanks. In some embodiments, the two LC tanks may beactively coupled together. The complex LC tank may include more than twoLC tanks. Advantageously, LC tanks of the complex LC tank may have ahigh impedance. Additionally, the amplifier and the complex LC tank maybe embodied in a deep-submicron CMOS node.

A complex-filtered form of the intermediate signal is fed back, to theamplifier through a loop that includes the complex LC tank to generatethe resonating signal in a step 440. The complex LC tank providescomplex filtering for the resonating signal. In some, embodiments, thecomplex LC tank provides 4^(th)-order complex filtering.

After feeding back the complex-filtered signal, the resonating signal isprovided by buffering the intermediate signal in a step 450.Advantageously a signal having the largest amplitude is used for theoutput resonating signal. In certain embodiments, for example, theillustrated embodiment of FIG. 1, the intermediate signal is thelargest-amplitude signal. One skilled in the art, however, willunderstand that another signal besides the intermediate signal may beused. If an effective total node capacitance is large enough and aninput impedance of a subsequent stage is sufficiently large, bufferingof the intermediate signal may not be needed.

After providing the output resonating signal, the method 400 ends in astep 460. Thus, the resonating signal can be generated in a continualloop. Of course, one skilled in the art will understand that thecontinuous generation of the resonating signal can end by terminatingany of the steps 410-440. For example, generation of the resonatingsignal can be terminated by removing the input signal or power supply tothe amplifier allowing the oscillation to die away.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. An n^(th)-order oscillator system for providing an output signal, nbeing at least three, comprising: an amplifier configured to provide anintermediate signal; and a feedback loop including an n^(th)-ordercomplex LC tank and configured to generate said output signal by feedingback a complex-filtered form of said intermediate signal at least one ofits resonating frequencies to said amplifier.
 2. The oscillator systemas recited in claim 1 wherein said complex LC tank comprises at leasttwo LC tanks.
 3. The oscillator system as recited in claim 2 whereinsaid complex LC tank comprises multiple LC tanks actively coupledtogether.
 4. The oscillator system as recited in claim 1 wherein said nis four.
 5. The oscillator system as recited in claim 1 wherein saidoutput signal is at a single oscillating frequency.
 6. The oscillatorsystem as recited in claim 1 wherein said oscillator system is embodiedin a deep-submicron CMOS process technology.
 7. A method of providing anoscillating signal, comprising: providing an intermediate signalemploying an amplifier; and generating said oscillating signal byfeeding back a complex-filtered form of said intermediate signal atleast one of its resonating frequencies to said amplifier through a loopthat includes an n^(th)-order complex LC tank.
 8. The method as recitedin claim 7 wherein said complex LC tank comprises at least two LC tanks.9. The method as recited in claim 8 wherein said complex LC tanks areactively coupled together.
 10. The method as recited in claim 7 whereinsaid oscillating signal is at a single resonating frequency.
 11. Themethod as recited in claim 7 wherein said amplifier and said complex LCtank are embodied in a deep-submicron CMOS process technology.
 12. Amethod of providing a resonating signal, comprising: providing anintermediate signal employing only one amplifier; and generating saidresonating signal by feeding back a complex-filtered form of saidintermediate signal to said amplifier through a loop that includes ann^(th)-order complex LC tank, n being at least three.
 13. The method asrecited in claim 12 wherein said complex LC tank comprises at least twoLC tanks.
 14. An n^(th)-order oscillator system for providing an outputsignal, n being at least three, comprising: only one amplifierconfigured to provide an intermediate signal; and a feedback loopincluding an n^(th)-order complex LC tank and configured to generatesaid output signal by feeding back a complex-filtered form of saidintermediate signal to said amplifier.
 15. An n^(th)-order oscillatorsystem for providing a resonating signal, n being at least three,comprising: an amplifier configured to provide an intermediate signal;and a feedback loop including an n^(th)-order complex LC tank andconfigured to generate said resonating signal by feeding back acomplex-filtered form of said intermediate signal to said amplifier. 16.A method of providing a resonating signal, comprising: providing anintermediate signal employing an amplifier; and generating saidresonating signal by feeding back a complex-filtered form of saidintermediate signal to said amplifier through a loop that includes ann^(th)-order complex LC tank, n being at least three.
 17. Acommunications system, comprising: a receiver; and an n^(th)-orderoscillator subsystem for providing a resonating signal to said receiver,n being at least three, including: only one amplifier configured toprovide an intermediate signal; and a feedback loop including ann^(th)-order complex LC tank configured to generate said resonatingsignal by feeding back a complex-filtered form of said intermediatesignal to said amplifier.
 18. A communications system, comprising: areceiver; and an n^(th)-order oscillator subsystem for providing aresonating signal to said receiver, n being at least three, including:an amplifier configured to provide an intermediate signal; and afeedback loop including an n^(th)-order complex LC tank and configuredto generate said resonating signal by feeding back a complex-filteredform of said intermediate signal to said amplifier.
 19. A communicationssystem, comprising: a receiver; and an n^(th)-order oscillator subsystemfor providing a resonating signal to said receiver, n being at leastthree, including: an amplifier configured to provide an intermediatesignal; and a feedback loop including an n^(th)-order complex LC tankwherein said complex LC tank is configured to generate said resonatingsignal by feeding back a complex-filtered form of said intermediatesignal at least one of its resonating frequencies to said amplifier. 20.The communications system as recited in claim 19 wherein said complex LCtank comprises at least two LC tanks.
 21. The communications system asrecited in claim 20 wherein said LC tanks are actively coupled together.22. The communications system as recited in claim 19 wherein said n isthree.
 23. The communications system as recited in claim 19 wherein saidresonating signal is at a single oscillating frequency.
 24. Thecommunications system as recited in claim 19 wherein said oscillatorsubsystem is embodied in a deep-submicron CMOS process technology. 25.The communications system as recited in claim 19 further comprising atransmitter coupled to the receiver.